Cooling of vacuum devices



July 4, 1944'.

' c. w. HANSELL 2,352,893

' COOLING OF VACUUM DEVICES Filed Dec. 18, 1941 2 Sheets-Sheet 1 F/lg. IN Z 1 RAD/0 c TRANSMITTER THROTTL/NG RADIATOR 340/0 TRANSMITTER INVENTORCLARENCE W HANSELL ATTORNEY y 1944- c. w. HANSELL ,8 4 COOLING OF VACUUMDEVICES Filed Dec. 18, 1941 2 Sheets-Sheet 2 POWER SUPPLY V CONTROLCONNECT/0N5 I 70 OTHER POWER SUPPLY CIRCUITS INVENTOR CLARENCE M-INA/SELL ATTORNEY Patented July 4, 1944 UNITED STATES PATENT OFFICE g.

' ooomno F VACUUM DEVICES Clarence Hansell, Port Jefferson, N. Y.,assignor to Radio Corporation of America, a corporation of DelawareApplication-December 18, 1941, Serial No. 423,448

' 3 Claims. (01. 250-275) This invention relates to the cooling ofvacuum tubes and to a system with means for controlling both temperatureand the temperaturedistribu- An object of this invention is'toreduce'electrical flash-over of high' vacuum'devices when they areoperated atrelatively' high electrical potentials.

Another object of this invention is to simplify and provide moreefiicient cooling of vacuumdevices including those of the mercury vaportype and also to provide greater economy in operation.

Still another object of this invention is to 'provide means in which thecooling fluid temperature of vacuum tubes as used; for radiotransmitters will be maintainedat a temperature which is higher than thenormal room temperature so that the tube envelope will remain at a lowertemperature than the tube electrodes.

Still further objects of this invention is to provide cooling meanswhich embraces a water cooling system and an air cooling system,separate, or a combination of both.

A further object of this invention is to maintain the anodes of a vacuumtube at an elevated temperature at all times with respect to the normalroom temperature, so that the amount of stretching and compression ofthe envelope material is greatly reduced, particularly when such vacuumtubes have metal-to-glass seals. The elevated temperature reduces thenumber and range of temperature cycles caused by starting and stoppingoperation. The reduction in temperature cycling at the seals will notonly reduce cracking and failure at the seal, but will reduce gasrelease where the metal material in and near the seals is repeatedlystretched within its elastic limit, particularly when the metal employedwithin the seals is a material having a relatively low elastic limit,such as for examplecopper.

A feature of this invention is the means to maintain a high anodeoperating temperature by providing means for increasing the pressure ofthe cooling fluid sufiiciently to raise its boiling point above that ofthe anode temperature.

Another feature of this invention is to maintain an elevated temperatureon the cathodes of the vacuum device by having a low heating currentcontinuously flowing through the cathodes when they are in a normalinoperative state.

For many years one of the most frequent causes of failure in operationof high voltage vacuum tube amplifiers has been fiashover or shortcircuiting between the operating elements. Flashover dilficultiesincrease very rapidly when potentials exceeding 10,000 to 15,000 voltsare employed and become so great at higher potentials that substantiallyno operation of 5 vacuum tube amplifiers is carried on at potentialsexceeding 20,000 volts. In fact, radio transmitter vacuum tubeamplifiers have been substantially limited to operation at potentialsbelow 16,000 volts because experience all over the world hasdemonstrated that use of higher potentials results in rapid loss ofreliability and early tube failures due to fiashover. This limitation ofoperating potential is practiced in spite of the fact that higherpotentials, if they could be used, would permit higher efilciencies andhigher power outputs from the amplifiers.

When using tubes of the UV-207 type, for example, I have found thatinternal fiashover is a frequent cause of trouble which results inservice interruptions and in the loss'of a considerable number of tubes.relatively high potentials of 10,000 to 15,000 volts for considerableperiods and then, without warning, will flash over. If the fiashoverdoes not destroy the filament, or cathode, then after a fiashover, thetube often will operate normally again and, for a time, can withstandhigher potentials than before. However, after an interval of timeranging from days down to hours or minutes, the tube will usually flashover again. The tubes seem to flash over after time intervals and thesetime intervals between fiashovers decrease in length successively untilthey become so frequent that the tube has to be removed, if thefiashover had not already destroyed the filament.

This tube fiashover is known all over the world, by those skilled in theradio art, as the Rocky Point effect because it was first found andrecognized as a fundamental problem during the course of research anddevelopment being carried out at Rocky Point, Long Island, New York.

It is my belief that the Rock Point effect in high vacuum tubes is verylargely due to the presence of migratory gas which is normally condensedor absorbed on the inner tube surfaces and which for that reason is notdetected by the usual gas tests given to the tubes. This gas slowlytransfers itself from one surface to another, alwaystending toaccumulate upon the coolest surface. The rate of transfer from onesurface to another may be very slow, perhaps like the transfer of icefromone place to another which can occur in closed vessels having adifference in temperature even though all temperatures in the vesselsare below the freezing point. In other words, I picture the migration ofgas in Tubes frequently operate at v the tubes lighted during idleperiods frequently helps to eliminate flashover when anode-andgridpotentials are applied again. I believe this may be very largely dueto heat of thelighted filament preventing or: reducing migration of gasback to the active tube surfaces. According to my invention, it istherefore desirable to keep the cathodes of the tubes heated at alltimes, though the heating may b reduced during idle periods.

In water cooled tubes as customarily operated, the anode is often muchcooler than other parts,

particularly just after power is removed. As aconsequence, there is acontinual force, due to temperature difference, tending to make the gasaccumulate on the anode surface. The hotter we allow the other parts ofthe tube, such as the glass portion of the envelope, to become, and thecolder we operate the anode, the greater is the rate of transfer of gasto the anode sur face.

This surface, in operation, is subjected to intense electron bombardmentwhich tends to vaporize the gas and to ionize it. The ions strike thegrid and cathode, particularly th grid, often with sufficient force toknock out both electrons and metal molecules. Often the ion andsecondary emission grid current has been observed to lower or evenreverse the normal grid current fiow, forcing the grid potentialmomentarily positive while the anode potential is high. A burst of gasand metal vapor from the anode and other parts resultsra-nd the tube.flashes over. If the flashover has not destroyed the filament, it isvery often found that, immediately following the flash, the tube willoperate normally or even withstand much more than normal cur-,

rent potential. I believe this is because the flashover has cleaned. gasoff. the anode. and other parts, causing it to' be transferred toinactive surfaces. However, when the anode is kept. cold, the gasmigrates back due to temperature difference and conditions arere-established for another fiashover. Flashovers may be repeated atintervals ranging from seconds up to days or weeks, depending upon thequantity and nature of the gas, the temperature distribution and themean operating temperature.

It seems probable that the gas which causes fiashover is not always orexclusively made up of molecules of material we commonly list as gases.I believe metal molecules may take part in a migration process or at anyrate tend to settle most on cool surfaces. Evidence of this is oftenobserved in glass envelope vacuum devices in which an internal coatingor blackening takes place usually on the coolest surfaces but not on hotsurfaces. It seems probable that a cold anode tends to accumulate metalvapor from the filament and that the vapor forms sub-microscopiccrystals and sharp protrusions which are easily revaporized by electronbombardment. Presence of thorium in the tungsten filament, a

common impurity, may result in thorium accumulations on a cold anodesurface.

Regardless of the detail nature of the causes of flashover, I believe itis conclusively proven that one means of reducing the prevalence of thephenomenon is to control the temperature distribution of the tubes tokeep gas away from the active surfaces which are subjected to electronand ion bombardment. That is, the surfaces of active parts should bemaintained at higher temperatures than inactive parts.

One means according to this invention for maintaining higher anodetemperatures in the case of water cooled tubes is to raise thetemperature of'the cooling fluid. If the cooling fluid is water; whichis customarily used, increasing the water temperature tends to reducethe permissible anode-power dissipation because of smaller temperaturerise permissible before boiling starts. I may overcome this difficultyby subjecting the cooling system to an increased pressure suificient toraise the boiling point of the water above the anode temperature.

Another means for maintaining higher anode temperature is by an aircooled system including a circuit arrangement for partially heating.the. surfaces of the active parts during idle periods.

This invention will best be understoodby referring to theaccompanyingdrawings, in which:

Fig. 1 is a fluid arrangement of'asystem of. this. invention;

Fig. 2 is another arrangement. of this invention similar to that of.Fig. 1 exceptv that an elevated reservoir is provided; and

Fig. 3 is a. powerv controlled circuit for the system of this invention.

Referring now in. detail. to Fig. 1 of the drawings, wherein one of. thesimplest possible. ways of operatingthe cooling fluid and therefore theanodes of the water-cooled vacuum tubes at higher temperature, isillustrated, in this figure there is indicated a radio transmitter Iwhich includes Water-cooled vacuum tubes, only one of which has beenshown and. indicated as 2.

The tube includes anode 2A cathode 2B.and.

grid 20. Cooling water for the tubes is circulated in a, closedcirculating. system comprisinga water pump 3, piping. 4,. an operatingvalve. 5;,

a throttling valve 6,.radiator T, and athermostat conventional mountingand.

8. Unlike the cooling systems of the prior. art, the anode. is mountedso as to be at a higher elevation than that of the tube envelope, andthe water pump for the system is chosen to provide higher than the usualpressure so that the boiling point of the water is raised. The increasedpump pressure, after the fluid passes through the vacuum; device 2, isabsorbed by placing a' resistance to the flow at some point on theoutlet; side of the connection to the vacuum tubes, either by means of arelatively small connection. or preferably.

by the adjustable throttling valve 6. The extra pump pressure appearsasan. increase in pressure of the water in the vacuum tube: coolingjackets water temperature may be increased about 34.4

centigrade without increase in the probability of boiling. at the vacuumtube anode surface.

In order that the temperature of the cooling water will always bemaintained at an elevated temperature, the thermostat 8 is employed forcontrolling the flow of water through the radiator 1. This thermostatmay be similar to those commonly used in automobiles to control theoperating temperature of water circulated through the engines. Then, ifwhen all the water flows through the radiator an amount of power equalto or greater than the power carried away from the radio transmitter bythe cooling water can be dissipated by the radiator, I may maintainsubstantially constant elevated temperature of the circulated water.

If the'transmitter is shut down and remains idle, the thermostatautomatically closes off circulation through the radiator and only asmall amount of electrical power in addition to water flow frictionlosses is likely to be needed to keep the circulating water up tooperating tempera ture. I propose to supply this power by dissi patingelectrical energy in the tubes even when the transmitter is idle.Usually it will be sufficient to keep a reduced amount of power in thevacuum tube filaments but power may be dissipated in the grid and anodealso with still better results in controlling gas distribution, ifdesired.

The radio transmitter should be provided with air blowers (not shown)which maintain relatively low temperatures of the glass and otherrelatively inactive portions of the tubes so that these parts will bemaintained at all times at lower temperatures than the active parts.

The arrangement shown by Fig. 2 is somewhat similar to that of Fig. 1,except that it does not require increased power to drive the circulatingpump 3, for the reason that a standpipe 10 or an elevated reservoirwhich is located substantially above the circulating system provides themeans for increasing the pressure, which when full of water subjects thewhole circulating system to increased pressure.

Other methods of increasing the pressure would be to connect the coolingsystem to a source of water supply, such as, for example, a public waterworks system, in which a higher pressure is maintained than the normalpressure needed for water circulation. Such a system would automaticallyreplenish any leakage in the system and prevent the possibility offailure of r the cooling system due to loss of pump priming.

In operating conventional water cooled tubes, it is customary to mountthe tubes with the glass ended portions up. With this arrangement, whenelevated cooling liquid temperatures, or air cooling fins are used,there is a tendency for heated air to rise up around the glass portionsand to increase the glass temperature. Under these conditions,continuously operated forced air cooling systems should be employed toforce down the glass temperatures to less than the anode temperature atall times. I have found that it will help to reverse the customary tubemounting method by placing the glass end down. In this case, it may bepossible to omit forced draft cooling, particularly while the tube isnot in use, because natural convection may provide sufficient coolingfor the glass during these periods.

In Fig. 3, there is shown a portion of the power control circuits'of aradio transmitter employing vacuum tubes of the 'type designed for watermay. be fused at l8 or connected in series with.

a circuit-breaker (not shown). Switch ll isno'rmally kept closed at alltimes when the transmit-. ter is in service or available for service.The,

tubes l9 and .20' each includes an anode 20A, cathode 20B. and grid 200.Power inputto the cathodes of the, tubes l9 and 20 is maintained whilethe transmitter is not in service; :This is. accomplished by anyadjustable impedance;

preferably that of a variable or adjustable rea'c-. tor. When the tubesI9 and 20 are to be placedin active operating service, the stop-startcontrol switch 22 is closed, thus causing ajcontactor relay 23 to close.Contactor relay 23 is provided with a plurality of contacts 24 whichclose circuits for applying other potentials, such as gridbias and anodepotentials. Closing the stopstart control switch 22 also starts the fansor blowers 25, 26 which force a rapid flow of air through the anoderadiators so that the high anode dissipation during normal running canbe accomplished with adequate air cooling, as described in more detailin the above mentioned Finch patent, particularly that portion relatingto Fig. '7 of the patent.

In the arrangement shown by Fig. 3, cool ingoing air passes over theglass envelope surfaces of tubes l9 and 20 and maintains these surfacescooler than the anodes. It is desirable to employ auxiliary blowers orfans, conventionally represented by 21 and 28, to force a current of airintimately in contact with the cathode stems, seals and tubulationsaround the grid leads where considerable heat may appear due toconduction from active portions of the tube and also to highfrequencyresistance and dielectric losses. In the practice of this invention, allportions of the glass should be maintained at a temperature well belowthat at which electrolysis in the glass becomes a factor in tubeoperation and life and low enough to cause the migratory materialsinside the tube to accumulate on the glass surfaces, particularlysurfaces removed from metal parts, seals and high potential gradients.

An incidental advantage of maintaining the anodes at an elevatedtemperatur at all times is that stretching and compression of envelopematerial, particularly at the metal-to-glass seals, is reduced byreducing the number and temperature range of temperature cycles causedby starting and stopping operation. It is my belief that reduction intemperature cyclin at the seals will itself not only reduce cracking andfailures at the scale but reduce gas release Where the metal material inand near the seals is repeatedly stretched beyond its elastic limit,particularly when the metal is a material having a relatively lowelastic limit, such as copper.

The principles I have described for controlling the distribution ofcondensed volatile materials in high vacuum tubes apply even moreforcefully in the case of mercury rectifier tubes and like devices inwhich case the presence of 1iquid mercury condensed on the surfaces inunwanted places, due to incorrect temperature distribution of thesurfaces, is often plainly visible. When suitably designed, these lowpressure mercury de.

Power for the transmitter.

vicem. employ; thm expedients: described: forrusewith;-hiah;vacuumtimes.v

whilezonlw-aa few: modifications; of this inventiom are.- ShOWIL; to; hedistirmtly5= understood.-

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What is: claimedis:

L Apparatus. for; reducing; the-tendency to: flash; over by avacuumtu'be, havingganoda and. cathode electrodes, com-prisinga; jacket for;the: anode. eithetube, a. pumr and connections; for circulating: water;through; thexjacket under pres-: sure; means for maintaining: thetemperature, above the normal boiling temperature at the: water; andmeans fordissipating electrical err-- ergy i'n someot the electrodesmfsaid tube during idle operating-periods-of said tube.

22 Apparatus for reducing: the tendency to flash over by a vacuum tubehaving anode andcathode electrodes, comprising a jacket for theanode-ofthe tube, a pump and connections for circulating water throughthe-jacket underpressure" for maintaining the temperaturesubstantiallyabove thenormal boiling-i temperatureof the water; meansfor dissipatingelectrical ener- 9Y in" some: of: the: electrodes offsaidtube: during:

idl operating periods of sat-(L tube, and! anthem mostaticallycontrolledscooling device associated Withs said connections.- ionincreasing. the cooling; Ofi the-waiter: duringcperiodsc ofi operationof said 150% ofhthe; time, a pump and pipaconuections; an

operatin zvatlve-in the, inlet water pipe. for saidjackemathrottlingjvaluein the-outlet water pipe, aradia-tor. flowconneeted across a sectionof the outlet Dining on the; sidaoi thethrottling valve removed irom the-tube and a. thermostaticallycontrolled valve for controlling water. flow throughsaid radiator forcirculating water through the -jacket.under pressure. for maintainingthetemperaturesubstantially above the. norl ma-l=boilingtemperature.ofthe Water, and means,

for. dissipating electrical; energy inv some of the electrodes ofsaidltube during idle operatingperiods of: saidvtube.

CLARENCE W. HANSELLL

